Differential mobility analyzer's (DMAs) are devices that are known for their use both in the laboratory and commercially. These devices seek to detect and analyse substances that are discriminated on the basis of the different ionic mobility of the charged particles.
A charged particle subjected to an electric field is accelerated in the direction of the field. If the ionized particle is immersed in a fluid, there is a resistance to the movement which prevents the particle from accelerating indefinitely, rather that it quickly attains a limit speed due to the balance between the electrical force that makes it accelerate and the resistance to movement caused by the fluid. This situation of balance establishes a value for the limit speed per unit of electric field [m2/Vs], called the ionic mobility limit, which mainly depends on the specific size and configuration of the charged particle, on the dynamic viscosity of the fluid, as well as on the strength of the electric field.
On the basis of this phenomenon, differential mobility analyzers establish an electric field with two electrodes in an area normally known as the analysis area, crossed by a cross flow in stationary conditions.
A charged particle that is injected into an electrode tends to travel to the other electrode due to the action of the electric field; nevertheless, the presence of a cross flow drags the particle in such a way that it will not impact following the line of the electric field, but at a point downstream.
The point of impact is different depending on the type of particle, since the mobility is the property that allows the discrimination of the substance of interest.
The resolution of this type of device depends, as described in U.S. Pat. No. 6,787,763, on the degree with which the turbulence in the cross flow is minimized, as well as of the Brownian diffusion.
The presence of turbulence causes fluctuations with respect to the average field of speeds that disperse the charged particle's trajectory and, when the average free travel of the particle is high due to the presence of reduced pressures or high typical residence times, the effects of the Brownian movement are greater.
The dimensionless Reynolds and Peclet numbers, defined as:Re=L·v/v ; Pe=L·v/D; Where v is a typical speed, L a typical length, v is the kinematic viscosity and D the molecular or Brownian coefficient of diffusion.
All these effects, turbulent diffusion and Brownian dispersion, are fully described and it has been proven that they depend on the Reynolds and Peclet numbers; and that these must be as high as possible in order to increase the resolution. Although it is known that, in general, for the flow in conduits, a Reynolds number of around 2000 is a critical value, above which the flow is turbulent, under certain conditions it is possible to maintain the laminar flow above this Reynolds value or with a small level of turbulence.
Mainly, the presence of favorable gradients of pressure that are sought for in all areas of the conduit, maintaining the boundary layer attached to the walls without its separation, preventing it from growing unnecessarily and eliminating the presence of disturbances (vibrations, roughness) that could release instabilities and increase the level of background turbulence.
The references cited in the background section of U.S. Pat. No. 6,787,763 are included for reference, noting that the said patent claims to reach Reynolds numbers in the range of 105. The geometry that is used in this analyzer is cylindrical, where there are factors of imprecision which cannot be avoided:                The cylindrical geometry has a central rod with cylindrical symmetry on which coaxiality has to be ensured and, given that this rod consists of more than one piece, its thinness and the machining errors of each piece and of the seatings accumulate, prejudicing coaxiality. This type of configuration requires at least five adjustments to achieve coaxiality. The lack of coaxiality, however small it may be, has an important effect on the electric field, which is very sensitive to this factor.        The use of cylindrical geometries does not offer any absorption of coaxial vortices in the duct which could induce oscillations in the flow.        In this type of device, once knowing the degrees of expansion and contraction to be normally high, being important sources of turbulence.        In current devices, the perimeter feed is carried out non-uniformly, which means that the reading conditions do not match a correct condition of cylindrical symmetry. The cross flow feeds in this type of device undergo sudden expansions that are not always stabilised down stream.        
The long length of the inlet mouth of the analyzer and the use of very small accelerations are described as inconveniences in the section of this patent dedicated to the state of the art.
In U.S. Pat. No. 6,787,763, use is made of the cylindrical configuration in which the inconveniences of the flat analyzer described in U.S. Pat. No. 5,869,831 are said to have been overcome.
It must be said that, although a flat analyzer is used in U.S. Pat. No. 5,869,831, both the injection or insertion of charged particles and the extraction are carried out via holes. It is enough to consider that the presence of instabilities in the flow or in the electric field could cause a very large adjustment problem, since the trajectory does not avoid three-dimensional effects in the direction perpendicular to the plane defined by the electric field and in the main direction of the cross flow. These deviations must also be controlled so that the adjustment of these factors causes the treatment of this analyzer to be three-dimensional in practice and not two-dimensional even though planes are used.
Although there are classifiers with configurations that are very close to two-dimensional behaviours with laminar flow conditions in the field of aerosols, these are made for particles that are injected with a secondary flow that causes a mixing layer that induces turbulence and a three-dimensional aspect, as well as an important change in the original profile of the speeds of the cross flow. Likewise, this type of device works with pressures that are lower than atmospheric pressure and with speeds in the subsonic or incompressible range.
Publication number WO2004048924 describes a method and apparatus for carrying out an ionic mobility spectrometry Use is made of a cross flow with an electric field. An ionizer injects the ions into the working volume perpendicular to the direction of the electric field, contrary to normal practice in a DMA, in such a way that the particle undergoes a double drag: one drag in the direction of the flow and which is in the same direction as that of the entry of the ionized particles, and a perpendicular drag due to the electric field. The combination of the two forces causes a trajectory that is, in principle, curved which depends on the ion's electrical mobility to reach a point that is more or less further away.
The spectrum readings are carried out in a vectorial charge sensor that provides different values for the deposited charge according to the incidence point. Given that the measurements are made within a specific time period, it is necessary to reset the instrument to zero before carrying out each test.
In all the backgrounds considered, the DMAs analyzed place the emphasis on their internal configuration, but not on upstream and downstream flow conditions. This invention includes a closed-circuit, pressurized, aerodynamic wind tunnel that prolongs the internal design of the analyzer in such a way that all the components involved in this flow are involved. The quality of the flow obtained is one of the main reasons that cause the analyzer's resolution to be notably higher. So much so that, for the first time, it has been possible to carry out measurements of particle mobility in the sub-nanometer range.
Therefore, this invention involves various improvements both overall and specific that increase the resolution and measurement range of the analyser, as well as other properties such as response and analysis speed, simple maintenance, efficiency, analysis capability in the sub-nanometer range and sensitivity.